The present invention resides generally in the field of the preparation of tetrabromophthalic diesters by the reaction of tetrabromophthalic anhydride with polyhydric alcohols and with alkylene oxides.
The study of hydroxyl-terminated derivatives of tetrabromophthalic anhydride as flame retardants has been ongoing for some time now. Tetrabromophthalic anhydride (TBPA) has a high bromine content (68.9%), by which it or its derivatives impart flame retardancy to plastic systems such as urethanes, especially urethane foams.
Because diesterification of TBPA is highly difficult using conventional esterification techniques, the art has turned to a more simple method which involves reacting TBPA with a polyhydric alcohol (e.g. glycols) to form the half-ester and with an alkylene oxide such as propylene oxide or ethylene oxide to form the diester.
The product of this reaction, referred to herein as a tetrabromophthalic diester (TBP-diester) composition, is a viscous material which has traditionally been characterized by its viscosity, bromine content and hydroxyl number. The bromine content of these TBP-diester products typically falls in the range of about 25% to about 50%. Hydroxyl numbers and viscosities vary widely with the particular reactants and reaction conditions used, but it may be generally stated that lower viscosities and higher hydroxyl numbers are desired for most applications.
Perhaps just as important as the specific bromine content, hydroxyl number, and viscosity of a TBP-diester product is the ability to consistently produce a product having these values within a specified range. End-users of these TBP-diester flame retardants optimize polymeric formulations into which the TBP-diesters are incorporated based on their characterizing properties. It is thus of critical importance that the flame retardant products be of a consistent quality from batch to batch. A batch falling outside specifications will be unsuitable to end-users and, unless it can be recovered by further processing, may have to be discarded. Needless to say, this result is highly costly. Further, attempts to recover out-of-specification batches are themselves expensive, and often fail.
For further general background information on the subject, reference can be made to patent and other literature, including U.S. Pat. Nos. 3,455,886; 3,459,733; 3,585,185; 3,642,646; 3,565,812; 3,637,543; and to Pape, P. G. et al., "Tetrabromophthalic anhydride in Flame-Retardant Urethane Foams", Journal of Cellular Plastics, Nov., 1968.
In light of this background and a continued demand for tetrabromophthalic diesters for use as flame retardants, there exists a need for processes which may be employed to provide TBP-diesters having consistent qualities from batch to batch. The present invention addresses this need.